Investment casting process

ABSTRACT

A method of manufacturing one-piece composite drill bits or coreheads suitable for drilling or coring petroleum wells or in mining. A shell (32) of hard wear-resistant and erosion-resistant material is formed by investment casting in a finished form requiring nominal or no finish shaping. A shank (44) of machinable material is then cast inside the shell, in conditions which cause fusion bonding of the two materials to form a one-piece composite drill bit (42) or corehead by the two-stage casting procedure. The shank (44) is finish machined to form a connection for attachment to a drill string. Pockets (20) for cutter inserts and gauge protectors can be pre-formed to final dimensions in the hard shell (32). The investment mold for the shell (24) allows the hard material to be cast in the required shape with little or no final shaping. The method enables drill bits and coreheads to be manufactured at relatively low cast by eliminating most or all skilled manual finishing operations.

This application is a continuation of application Ser. No. 08/052,862filed Apr. 26, 1993 now abandoned, which is a continuation of Ser. No.07/669,420 filed Apr. 2, 1991 now abandoned.

This invention relates to a method of manufacturing petroleum/miningdrill bits/coreheads with synthetic and natural diamond materials byutilising investment casting methods.

Current methods for producing drill bits/coreheads utilise a matrix or asteel body.

In the matrix type, tungsten carbide powder matrix is formed in a thickshell around a steel inner core which carries the threaded connection.The cutters are then brazed on to the pre-formed matrix shell.

This method is suitable since the tungsten carbide matrix is veryresistant to fluid erosion and abrasive wear, natural diamonds can beincluded in the matrix shell for gauge protection, and relativelycomplex shapes can be produced.

However, the method suffers from the disadvantages that possiblebreakdown of bond between the matrix shell and steel core may occur,manufacture of the graphite mould is precision work requiring highlabour input, and high cost due to quantity of carbide required.

Also, the differential of contraction between matrix shell or steel coremay cause cracking especially in the larger products and further, poorquality of the matrix body formed necessitates extensive hand fettling.

In the steel body type, the normal method of manufacture is by machiningfrom the solid using multi-axis milling machines and then hard-facingusing welding or spray metal techniques prior to the installation of thecutters. These cutters are either brazed in place or pressed intoprepared holes and held in place by interference fit.

The advantages of the steel body type are a single unit constructionwith no possibility of break-up due to bond failure or cracking, lowcost materials, and CNC multi-axis milling machine techniques give goodrepeatability for batch production.

However, the steel body type method is labour intensive, in that hardfacing has to be applied after machining, and any surplus hard facinghas to be hand-ground away from cutter pockets prior to installation.Also, the allowable complexity of shape is restricted by limitations ofmachining capabilities.

It has previously not been considered a viable solution to manufacturedrill bits/coreheads utilising investment casting techniques; the matrixand CNC machining approach being far more established and understoodthan this hitherto unknown method of manufacturing.

The accepted standard method of manufacturing an investment casting forindustrial products such as aircraft turbine blades and enginecomponents is as follows:

A master mould is manufactured to cast accurate wax males of the productrequired. The wax males are then coated with a ceramic material bydipping them in a slurry and then raining sand on the wet slurry. Thisis done a number of times, allowing the slurry and sand coating to drybefore re-dipping.

In this way, a thick coating of material is built up around the waxmale. The coated wax male is then furnaced to bake the coating and meltout the wax, thus creating an accurate ceramic mould of the product tobe cast.

Under normal circumstances, this method of manufacture would not be usedto produce a steel-bodied bit or corehead due to the fact that it wouldrequire subsequent hard facing after casting in order to withstand thefluid erosion and abrasive wear experienced downhole. The application ofthis hard facing by spray metal or welding techniques would cover ordamage the accurately-formed profile of the investment cast product thusspoiling the dimensional accuracy and therefore defeating the purpose ofusing this process in the first place.

It is an object of the invention to obviate or mitigate the abovedisadvantages by utilising the investment casting process in a novelmethod of manufacture to product a highly accurate and, if required,complex casting, which needs little refinishing prior to installation ofthe cutters.

According to a first aspect of the present invention, there is provideda method of casting a drill bit or corehead, said method comprising atwo-stage process wherein the first stage comprises forming a relativelyhard outer shell by investment casting, and the second stage comprisescasting a relatively less hard core within the outer shell in conditionswhich cause fusion bonding of shell and core, the outer shell havingsubstantially the final form of the outer part of the intended drill bitor corehead, and the core having at least principal features of thefinal form of the shank of the drill bit or corehead.

According to a second aspect of the present invention, there is provideda method of casting a drill bit or corehead, said method comprising thesteps of forming or providing at least the basis of a shank of the drillbit or corehead, the shank or proto-shank being of a relatively lesshard material, buttering the shank (or proto-shank) with weld materialor spray metal deposit of lower melting temperature than that of amaterial subsequently to form an outer shell of the drill bit orcorehead, forming or providing a ceramic mould of the bit head,suspending the pre-buttered shank or proto-shank in the ceramic mould,pre-heating the ceramic mould and shank (or proto-shank) to apredetermined casting temperature, and casting the relatively hardmaterial around the shank (or proto-shank) to be fusion bonded theretoand to form a relatively hard outer shell around the shank.

According to a third aspect of the present invention there is provided adrill bit or corehead, manufactured by the method according either tothe first aspect of the present invention, or to the second aspect ofthe present invention.

The method of the invention combines the advantages of both matrix andsteel bodied type production, substantially reducing the labour contentper manufactured unit, thus greatly enhancing the possibilities of massproduction.

In order to achieve a product which would fulfil the requirement of theindustry, it was necessary to devise a method of investment casting ahard bit body whilst retaining a tough machinable central core. This wasachieved by casting the bit body utilising investment casting methods.

In accordance with the invention, the drill bit/corehead is made by twoseparate casting stages in a two-part manufacturing process, the bodybeing cast in separate casts as follows:

Cast 1: to create a very hard and fluid erosion resistant outer shellwhich has the accuracy of outer form that the investment casting couldproduce.

Cast 2: to cast within this shell a central core which was tough yetmachinable, in such a way that fusion bonding of the two materials isachieved and the final casting is a single piece of materialincorporating a tough central core and having an outer casing of hardmaterial which is highly resistant to abrasive wear and erosion wear.

The purpose of producing the bit in a two-step casting is that the bitshank requires different properties to the bit head i.e. the bit headrequires to be resistant to abrasive wear and to be resistant to fluiderosion whereas the shank requires to be easily machinable and to havethe capability of withstanding high stress/fatigue levels.

These properties are not realistically achievable from one material.

The complex form of a drill bit head is difficult and expensive tomachine and therefore lends itself to the investment casting process.The bit shank on the other hand is less critical and can be sand cast orinvestment cast and machined to size at a later stage.

In addition to creating an investment cast drill bit/core head with thehard facing in situ, the internal hydraulic manifolding required todirect fluid to the nozzles in the bits used for cooling and cleaning,could be cast in situ in the second cast by installing thisprefabricated ceramic into the shell of the first cast and castingaround; thereby creating the bit complete with its manifolding in atwo-step casting process.

However, inclusion of the manifold may be omitted if so necessitated bythe design.

In carrying out this novel manufacturing process, a preferredpreliminary stage is to produce an accurate male wax model of the bithead to be cast. This can be achieved in a number of ways:

Method 1--it can be machined from the solid piece of wax attached to amandrel. (This is a particularly useful approach for prototyping orbatch production.)

Method 2--wax injection mould dies can be manufactured for theparticular component and injection mould wax males can be produced.(Suitable for mass production).

Method 3--a combination of methods 1 and 2 can be used i.e. injectionmould the basic shape and carry out minor machining on the wax. (Thisallows for greater flexibility for cutter and gauge protection slugpositioning while maintaining the advantages of relatively low cost massproduced waxes).

Method 4--wax injection mould can be produced for the bit in componentform and these mass produced component parts assembled at the wax stageto produce a variety of bits. (This allows for mass production of avariety of products at relatively low cost).

The preferred second stage of manufacture is to produce a ceramic mouldfrom the wax male which has been produced by one of the above methods.This may be done by the conventional investment casting method aspreviously described.

The preferred third stage is to make an investment casting by pouringmolten alloy into the prepared ceramic mould thus producing an exactcopy of the original wax male. This casting material should be highlyresistant to abrasive wear and fluid erosion in its cast stage e.g. thehigh-cobalt alloys such as stellite. The resultant casting preferablyincorporates all the cutter and gauge slug pockets to a high degree ofaccuracy. It may also include fluid porting and nozzle positionstogether with an internal attachment profile such as thread slots orkeyways.

The preferred fourth stage, after casting and cleaning, is to fit theinternal ceramic components into position within the hard shell andprepare the hard shell for the second casting operation. The shell fromthe first cast is therefore now set in a sand mould bed with the shankform created using sand or ceramic moulding techniques.

The preferred fifth stage is to pre-heat the combined mould before thesecond cast to such a level that it takes into account the temperatures,masses and specific heat values of the two materials being combined suchthat a percentage of the inner skin of the outer shell is melted down toform a fusion bond between the two materials. This will cause alloyingof the two materials causing fusion bonding to take place between thehard outer shell and the softer but tougher inner core material. Latentheat of fusion plays a major role in this process, ensuring that fusionbonding can take place without total melt-down of the shell. A suitablepre-heat temperature of the shell can be determined by taking intoaccount the relative masses and respective temperatures of the shell andthe material poured to product the inner core.

After this second casting process, a product will have been manufacturedwhich has an accurately-formed, hard wear-resistant outer shell fusionbonded onto a tough machinable inner core.

This will have particular application to the manufacture of drillbits/coreheads for the petroleum and mining industries.

It should also be noted that the manufacturing process described aboveis flexible and is capable of being reversed, by casting the hardmaterial around the shank as follows:

Step 1: form or provide a shank (or at least the basis of a shank, to befinished subsequently), by casting or by any other suitable process;

Step 2: butter the shank with weld material or spray metal deposit, oflower melting temperature than the material of the cast shell;

Step 3: form or provide a ceramic mould of the bit head;

Step 4: suspend this pre-buttered shank in the ceramic mould of the bithead;

Step 5: pre-heat the ceramic mould and shank assembly to correct castingtemperature; and

Step 6: cast the hard material around the shank to form a hard shell.

Embodiments of the manufacturing process will now be described, by wayof example, with reference to the accompanying drawings, in which:

FIG. 1 is a wax block cast onto an alloy mandrel ready for machining;

FIG. 2 is a wax shell of a typical drill bit head or crown, taken offthe mandrel after machining;

FIG. 3 is a cross-section of the ceramic mould for the first phase ofcasting;

FIG. 4 shows the second phase of casting with a manifold in position;

FIG. 5 shows a cross-section of the completed bit body, ready forinstallation of cutters and gauge protection slugs; and

FIG. 6 is a perspective view of the completed drill bit.

Referring first to FIG. 1, an alloy mandrel 10 has an attachment thread12 formed on one end. A wax block 14 (shown in ghost outline) is castaround the thread 12 to form an assembly ready for machining to shape.

FIG. 2 shows a wax shell 16 as typically machined from the block 14, andunscrewed from the thread 12 to leave an internal attachment thread 18.The cutter shell 16 has a four-bladed form, with pockets 20 on the bladeedges for subsequent mounting of cutter inserts, and side-face pockets22 for subsequent insertion of hard inserts to maintain cutter gaugeagainst diameter reduction by wear. As an alternative to being machined,the wax shell 16 could be formed by injection moulding.

A ceramic mould 24 (FIG. 3) is formed from the wax shell 16, the mouldincluding runners 26 and a riser 28 for the pouring in of liquid metal.The ceramic mould 16 is mechanically supported in a bed of sand 30during the first stage of the casting process.

FIG. 4 shows the second stage of casting, in which the first-stagecasting 32 (with risers removed) is placed against a ceramic shank mould34. A ceramic manifold insert 36 is placed within the casting 32 to forma manifold in the second-stage casting. The assembly of first-stagecasting 32 and shank mould 34 is mounted within and supported by sand 38held in a drum 40.

FIGS. 5 and 6 show the composite casting 42 resulting from the secondstage of the moulding process. The composite casting 42 includes a bitshank 44 fusion bonded to the hard first-stage casting 32 along a fusionbond line or zone 46. A central conduit 48 runs from a connector 50 onthe bit shank 44 through to a flow manifold chamber 52 and thence tonozzles 54, these passages being formed in the second stage of casting(FIG. 4) by the inclusion of the ceramic manifold insert 36. PDC cutters56 are mounted in the pre-formed cutter pockets 20 (FIG. 4) in the bladeedges, and hard slugs or inserts 58 are fitted in the pre-formed pockets22 outer edges of the blades, to act as gauge protectors.

The process of the invention has the advantage that highly accurateinvestment casting requires a minimum of hand grinding, machining etc,prior to cutter installation, thus substantially reducing labour contentinvolved in the standard method of producing drill bits/coreheads.

Fusion bonding ensures integrity of bond between the shank and bit head.The casting method allows for greater flexibility in the design of fluidporting, and in cutter and gauge insert installation. The inherentaccuracy of the casting process gives better quality control of cutterpockets and braze bond integrity due to the fine clearances achievable,giving good capillary action of the braze material and betterself-distribution.

Injection moulded wax ensures consistency of cutter positioning andtherefore of bit performance.

Thus, there has been described a method of manufacture which utilisesthe investment casting process to give the degree of accuracy requiredfor producing drill bits/corehead bodies, and enables the hard facing tobe applied in a two-part manufacturing process.

Modifications and variations of the above-described processes andproducts can be adopted without departing from the invention as definedin the appended claims.

I claim:
 1. A method of casting a drill bit or corehead having a cuttingpart and a shank, said method comprising the steps of providing a shankelement which forms at least the basis of said shank, the shank elementbeing of a relatively less hard material, buttering the shank elementwith metallic coating selected from weld material and spray metaldeposit each of lower melting temperature than that of a relatively hardmaterial subsequently to form an outer shell of the cutting part of thedrill bit or corehead, providing a ceramic mold of said cutting part,suspending the pre-buttered shank element in the ceramic mold,pre-heating the ceramic mold and the shank element to a predeterminedcasting temperature, and casting the relatively hard material around theshank element to form a relatively hard outer shell around the shankelement, said step of casting comprising the imposition of conditions onsaid casting step which cause the relatively hard material duringcasting thereof to produce localized melting of said metallic coating toproduce fusion bonding between said shell and said shank element.
 2. Amethod of casting a drill bit or corehead having a cutting part and ashank, said method comprising a two-stage process wherein the firststage comprises forming a relatively hard outer shell by investmentcasting, said outer shall having an outer surface and an inner surface,and the second stage comprises casting a relatively less hard corematerial to form a core within the outer shell, the outer shell havingsubstantially the final form of the outer part of the intended cuttingpart of the drill bit or corehead, and the core having at least theprincipal features of the final form of the shank of the drill bit orcorehead, said second stage further comprising imposing conditions onsaid casting process which cause the core material during castingthereof to produce localized melting of the inner surface of the outershell to produce fusion bonding between said shell and said core.